Consider a typical MANET with four mobile nodes 1, 2, 3, 4 as shown in figure. Nodes are acting as router where any node can transmit packets to other nodes, if they are in communication range. Node 1 is sending packet to node 3 via node 2. After some time node 2 starts moving away and not in the communication range of node 1. So the route breaks between node 1 and 2. In the mean time node 4 comes in the communication range of node 1 and a new route 1-4-3 is created and transmission of packets begins from node 1 to 3 via node 4.
The flash video on the right hand side shows how nodes are acting as routers in a mobile adhoc networks.